Mercury - Hg

Atomic number 80

Atomic mass 200.59 g.mol -1

Atomic number 80 Atomic mass 200.59 g.mol -1

Density 13.6 g.cm-3 at 20°C

Melting point - 38.9 °C

Boiling point 356.6 °C

Discovered by The ancients

Mercury is the only common metal which is liquid at ordinary temperatures. Mercury is sometimes called quicksilver. It is a heavy, silvery-white liquid metal. It is a rather poor conductor of heat if compared with other metals but it is a fair conductor of electricity. It alloys easily with many metals, such as gold, silver, and tin. These alloys are called amalgams.

The most important mercury salts are mercuric chloride HgCl2 (corrosive sublimate - a violent poison), mercuric chloride Hg2Cl2 (calomel, still used in medicine occasionally), mercury fulminate (Hg(ONC)2, a detonator used in explosives) and mercuric sulphide (HgS, vermillion, a high-grade paint pigment).

Applications

Mercury metal has many uses. Because of its high density it is used in barometers and manometers. It is extensively used in thermometers, thanks to its high rate of thermal expansion that is fairly constant over a wide temperature range. Its Its ease in amalgamating with gold is used in the recovery of gold from its ores.

Industry uses mercury metal as a liquid electrode in the manufacture of chlorine and sodium hydroxide by electrolysis of brine. Mercury is still used in some electrical gear, such as switches and rectifiers, which need to be reliable, and for industrial catalysis. Much less mercury is now used in consumer batteries and fluorescent lighting, but it has not been entirely eliminated.

Mercury compounds have many uses. Calomel (mercurous chloride, Hg2Cl2) is used as a standard in electrochemical measurements and in medicine as a purgative. Mercuric chloride (corrosive sublimate, HgCl2) is used as an insecticide, in rat poison, and as a disinfectant. Mercuric oxide is used in skin ointments. Mercuric sulphate is used as a catalyst in organic chemistry. Vermilion, a red pigment, is mercuric sulphide; another crystalline form of the sulphide (also used as a pigment) is black. Mercury fulminate, Hg(CNO)2, is used as a detonator.

Mercury in the environment

Mercury occurs uncombined in nature to a limited extent. It rarely occurs free in nature and is found mainly in cinnabar ore (HgS) in Spain, Russia, Italy, China and Slovenia. World production of mercury is around 8.000 tonnes per year. Mineable reserves are around 600.000 tonnes.

Mercury is a compound that can be found naturally in the environment. It can be found in metal form, as mercury salts or as organic mercury compounds.

Mercury enters the environment as a result of normal breakdown of minerals in rocks and soil through exposure to wind and water. Release of mercury from natural sources has remained fairly the same over the years. Still mercury concentrations in the environment are increasing; this is ascribed to human activity.

Most of the mercury released from human activities is released into air, through fossil fuel combustion, mining, smelting and solid waste combustion. Some forms of human activity release mercury directly into soil or water, for instance the application of agricultural fertilizers and industrial wastewater disposal. All mercury that is released in the environment will eventually end up in soils or surface waters.

Mercury is not naturally found in foodstuffs, but it may turn up in food as it can be spread within food chains by smaller organisms that are consumed by humans, for instance through fish. Mercury concentrations in fish usually greatly exceed the concentrations in the water they live in. Cattle breeding products can also contain eminent quantities of mercury. Mercury is not commonly found in plant products, but it can enter human bodies through vegetables and other crops, when sprays that contain mercury are applied in agriculture

Health effects of mercury

Metallic mercury is used in a variety of household products, such as barometers, thermometers and fluorescent light bulbs. The mercury in these devices is trapped and usually does not cause any health problems. However, when a thermometer will break a significantly high exposure to mercury through breathing will occur for a short period of time while it vaporizes. This can cause harmful effects, such as nerve, brain and kidney damage, lung irritation, eye irritation, skin rashes, vomiting and diarrhoea.

Mercury has a number of effects on humans, that can all of them be simplified into the following main effects:

Environmental effects of mercury

Mercury from soils can accumulate in mushrooms.

Acidic surface waters can contain significant amounts of mercury. When the pH values are between five and seven, the mercury concentrations in the water will increase due to mobilisation of mercury in the ground.

Once mercury has reached surface waters or soils microrganisms can convert it to methyl mercury, a substance that can be absorbed quickly by most organisms and is known to cause nerve damage. Fish are organisms that absorb great amounts of methyl mercury from surface waters every day. As a consequence, methyl mercury can accumulate in fish and in the food chains that they are part of.

The effects that mercury has on animals are kidneys damage, stomach disruption, damage to intestines, reproductive failure and DNA alteration

The greatest source of mercury in the biosphere is currently of human origin. Mercury is number three on the ATSDR 2011 Substance Priority List Two-thirds of the mercury entering the environment comes from man-made sources including industrial plants, coal burning, and incinerators, and the additional one-third enters from natural sources. Many former chlor-alkali facilities are major point sources of mercury to aquatic ecosystems and are currently designated Superfund sites. Mercury is released into the air or directly into waterbodies and makes its way into lakes and estuaries, where some of it settles to the bottom.

Mercury is considered a global pollutant capable of spreading far beyond its source area. Even the arctic, which has no known sources of mercury, harbors mercury contaminated fish, and recent studies indicate whales who feed in the arctic have high levels of mercury in their tissue. Lake, river, and estuary bottoms serve as the site for the transformation of mercury to methylmercury by bacteria living in the mud.

Methylmercury is the extremely toxic form of mercury that biomagnifies in aquatic food chains. It is a potent neurotoxin and the easiest form for animals to store in their tissues. It harms the brain, affecting memory, understanding, and movement. Studies have shown that mercury exposure in humans can result in developmental delays in children, motor impairment, cardiovascular effects, and in acute cases, death. Its effects have been studied in fish, whales, seals, and seabirds. Methylmercury binds to proteins and easily crosses cell membranes, including the blood-brain barrier and the placenta. Affected wildlife, such as loons, develop behaviors that ultimately reduce their survival and reproduction, and put the population at risk. Studies conducted on human populations have estimated that between 200,000 and 400,000 children in the United States alone are born each year with pre-natal exposure to methylmercury sufficient to put them at risk of neurologic impairment.

Mercury in fish

People are exposed to mercury mainly through eating fish and shellfish - and 95 percent or more of the mercury in fish is the more toxic methylmercury. According to EPA, fish fillets containing more than .3 parts per million (ppm) of methylmercury should not be eaten (Canada and the states of Maine and Minnesota suggest you avoid fish with .2 ppm). Fish caught in water with very low concentrations of mercury (less than 1 part per trillion) can nonetheless contain harmful levels of methylmercury. In some marine ecosystems, the concentration of methylmercury increases 10 million times as it makes its way up through the food web from microscopic algae to shark and tuna.

Mercury concentrations in fish from lakes and rivers throughout the United States now exceed the mercury levels that are cause for human and wildlife health concern. As of 2008, all 50 states and one territory have fish consumption advisories for mercury. In addition, all states on the Atlantic and Gulf of Mexico have coastal fish advisories. Although much of the scientific research on mercury in fish has focused on freshwater ecosystems, most Americans are exposed to mercury through seafood consumption. There are many questions to be answered about where the mercury in the fish that we eat comes from and what fish are safe to eat. Solutions to the complex problem of mercury pollution have been impeded by conflicting information on the sources, transport, and accumulation of mercury in the environment. Our program hopes to address these questions and therefore provide the scientific basis for solutions to this pressing environmental and human health issue.